1. Field of the Invention
This invention relates to a light amplification coupler which amplifies signal light and simultaneously branches it into a plurality of signal light beams.
2. Description of the Related Art
In an optical subscriber type system, it is increasing to distribute signal light of a CATV (cable television) or the like into a large number of light beams to transmit the same to individual subscribers. In such transmission, since the signal light intensity decreases as the signal light is distributed, the signal light is amplified by an optical amplifier first and then distributed using a multiple-branching coupler. Several types of optical amplifiers by which an optical signal is amplified directly are known. Of the known optical amplifiers, an optical fiber amplifier wherein a rare earth doped fiber is combined with pump light is used very frequently in recent years because it has such superior characteristics that it is free from a polarization dependency, that it is low in noise and that it exhibits a low coupling loss with a transmission line. Employment of optical fiber amplifiers allows a remarkable increase in transmission repeating distance in an optical fiber transmission system and distribution of an optical signal to a large number of signal light beams.
Referring to FIG. 1, there is shown a schematic view of a conventional signal light distribution system. Signal light is inputted to an input port 3 of an optical fiber 2. The signal light propagates in the optical fiber 2 and is inputted to an optical multiplexer 6. Meanwhile, also pump light from a pumping laser diode 4 is inputted to the optical multiplexer 6, and the signal light and the pump light are multiplexed by the optical multiplexer 6. The multiplexed signal light and pump light propagate in an Er doped fiber 8 which includes a core doped with Er. During the propagation, the signal light is amplified along the Er doped fiber by a well known principle. The amplified signal light is supplied to an optical filter 10, by which amplified spontaneous emission light (ASE light) and a remaining component of the pump light are removed therefrom. Thereafter, the signal light is branched into a large number of light beams by a branching coupler 12, and the branched signal light beams thereafter propagate in optical fibers 14a to 14e.
In this manner, in the conventional optical signal distribution system, an optical signal is first amplified by an Er doped fiber, and then the signal light is distributed into a large number of signal light beams using a branching coupler. For the branching coupler, a fusion type coupler formed from a plurality of optical fibers fused to each other or a waveguide type coupler is used. However, since an existing amplifier and an existing branching coupler are used in combination, this makes a factor of making reduction in cost and space difficult.
Further, an optical multiplexer for multiplexing signal light and pump light must be provided, and a construction for splicing a plurality of optical fibers and another construction for optically coupling the optical fibers by means of an optical prism is required. Further, the optical amplifier must be assembled by individually joining filters for removing ASE light which makes noise.